1. Field of the Invention
The present invention generally relates to a technology for driving a rotating-body by transmitting a rotation force from a rotation-driving source via a rotation-force transmission mechanism.
2. Description of the Related Art
In an electrophotographic-system image forming apparatus that forms an image by forming toner images on a surface of a photoconductive drum and transferring them to a recording sheet, for example, it is necessary to accurately match peripheral velocity of a photoconductive drum with a carrier speed of a recording sheet to transfer toner images formed on a surface of a photoconductive drum to a recording sheet without change.
When the photoconductive drum is rotated and driven, for example, by a DC motor, it is general on account of stabilization of rotation velocity and securement of a driving torque that the motor is rotated relatively at high velocity and the rotation velocity is reduced by decelerating means such as a gear reducer to drive the photoconductive drum. However, in this event, even if the motor as a rotation-driving source is rotated at a stable velocity, periodic variation occurs in rotation velocity of the photoconductive drum due to a difference in processing accuracy (an accumulated pitch difference concerning a gear, decentering of a rotating shaft, and the like) in a rotation force transmission mechanism that includes a gear. As a result, there is a possibility that a reproduced image is degraded.
Therefore, a rotating-body driving device is proposed to correct this velocity variation that, on condition that a motor is previously rotated at a certain velocity in shipment of an image forming apparatus, exchange of a photoconductive drum, or the like, the rotation force is supplied through a rotation force transmission mechanism to a rotating-body, and a periodic variation component in rotation velocity of the rotating-body is measured to store it in a memory, reads the periodic variation component from the memory, when using an image forming apparatus, and performs velocity correction in opposite phase to reduce velocity variation of the photoconductive drum (see Japanese Patent Application Laid-open No. 2005-312262).
As shown in FIG. 24, in the rotating-body driving device, a disk-shaped detection target body (encoder) 111 that includes a single slit 114 for detecting a reference rotational-position and a plurality of slits 113 (4 slits in this case) for detecting the other rotational positions is mounted around a rotating shaft 112 of a photoconductive drum, and a detector 117 that detects a rotational position of each of the slits that move along with rotation of the photoconductive drum is arranged opposite to the encoder 111. A motor is rotated at a certain velocity and a time difference of timing at which the detector 117 detects the slits 113 is detected. After a calculation, a periodic variation component is extracted, as shown in FIG. 25, the component is stored in a memory by corresponding to timing (home position) at which the detector 117 detects the slit 114 and then the slit 113. To correct velocity variation, when detecting the above home position, a periodic variation component is read from the memory based on a phase corresponding to the home position and velocity correction in opposite phase of the periodic variation component is performed so that, as shown in FIGS. 26A and 26B, periodic variation in rotation velocity of the photoconductive drum is controlled.
However, the slit 114 for detecting a reference rotational-position is mounted on the encoder 111 separately from the slits 113 for detecting a time difference in the rotating-body driving device. Therefore, for example, when the number of slits 113 is increased to enhance accuracy of detecting a time difference for accurate extraction of a periodic variation component, it is difficult to provide the slit 114. The slit 114 is a slit only to detect a home position. Therefore, whenever a slit is detected, it is necessary to have determination means to determine once, after the detector 117 detects a slit, whether the slit is for detecting a home position or for detecting correction data and to store only the slit that is determined as a slit to detect correction data in a memory, thereby increasing a load to process software.
Thus, the applicant of the application proposes a rotation detecting device that uses a slit that has a larger width for detection of both a home position and velocity variation by making one of slits 113 shown in FIG. 24 wider in a peripheral direction of the encoder 111, identifying passing of the slit that has a larger width based on a difference of a detection signal from the detector 117 caused by a difference in a width of a slit, counting the number of detection of ends of slits 113 in the peripheral direction (a front end in a rotating direction of the encoder 111) through the detector 117 from the time point, and detecting an end of a slit in the peripheral direction with respect to the number of counting the following slits before detecting the slit 113 that has a larger width (“4” in FIG. 24) as well as generating a home position signal (Patent Application No. 2005-266708).
However, the rotation detecting device identifies passing of the slit that has a larger width and then generates a first home position signal after a rotation of the photoconductive drum. Therefore, until a home position is detected after starting a motor and the photoconductive drum rotates once, correction of velocity variation is not started. It is required to reduce time to form a first copy in an image forming apparatus in view of energy saving and appliance with respect to a user. It is necessary, to meet the requirement, to form an image on a photoconductor in a possibly short time after start of a motor. However, it is impossible for the rotation detecting device to sufficiently meet the requirement of reducing time to copy.